Anaerobes are organisms that grow and reproduce without molecular oxygen and may even be injured or killed by its presence. They obtain energy through fermentation or anaerobic respiration, using inorganic or organic compounds other than oxygen as terminal electron acceptors.
Explanation
Obligate anaerobes lack the enzymes, such as catalase and superoxide dismutase, needed to detoxify reactive oxygen species and are harmed by exposure to air. They rely on fermentative pathways or anaerobic respiration to generate ATP. Facultative anaerobes can switch between aerobic respiration and fermentation depending on oxygen availability, while aerotolerant organisms do not use oxygen but possess protective enzymes that allow them to tolerate it. Microaerophiles prefer low oxygen levels but will grow poorly or not at all in atmospheric oxygen. Anaerobes inhabit environments depleted of oxygen, including deep soil, sediments, the gastrointestinal tract, and dental plaque. In these niches they play essential roles in organic matter decomposition and nutrient cycling.
Examples and Roles
Notable obligate anaerobic bacteria include Clostridium perfringens, C. botulinum and C. tetani, which produce potent toxins and cause diseases such as gas gangrene, botulism and tetanus. Bacteroides fragilis and other members of the gut microbiota are anaerobes that aid in digestion and help exclude pathogens. Strictly anaerobic archaea, such as methanogens, generate methane from carbon dioxide and hydrogen in the rumen of ruminants and in anaerobic digesters. Facultative anaerobes such as Escherichia coli and Staphylococcus aureus are ubiquitous and can grow with or without oxygen.
Because many anaerobes are part of the normal microbiota, disruption of ecological balance or introduction into sterile tissues can lead to infection. Culturing anaerobic bacteria requires reducing agents in growth media, exclusion of oxygen and specialized techniques such as anaerobic jars or glove boxes.
Related Terms: Obligate anaerobe, Facultative anaerobe, Aerotolerant, Microaerophile, Fermentation